The present invention relates to plasma arc torches and, more particularly, to an electrode for supporting an electric arc in a plasma arc torch.
Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treatment, melting, and annealing. Such torches include an electrode which supports an arc which extends from the electrode to the workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex flow of gas, and in some torch designs it is conventional to also envelope the gas and arc with a swirling jet of water.
The electrode used in conventional torches of the described type typically comprises an elongate tubular member composed of a material of high thermal conductivity, such as copper or a copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive element embedded therein which supports the arc. The element is composed of a material which has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (ev), which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the element is thus capable of readily emitting electrons when an electrical potential is applied thereto. Commonly used emissive materials include hafnium, zirconium, tungsten, and their alloys. The emissive element is typically surrounded by a relatively non-emissive separator, which acts to prevent the arc from migrating from the emissive element to the copper holder.
A problem associated with torches of the type described above is the short service life of the electrode, particularly when the torch is used with an oxidizing gas, such as oxygen or air. More particularly, the gas tends to rapidly oxidize the copper of the electrode that surrounds the emissive element, and as the copper oxidizes, its work function decreases. As a result, a point is reached at which the oxidized copper surrounding the emissive element begins to support the arc, rather than the element. When this happens, the copper oxide and the supporting copper melt, resulting in early destruction and failure of the electrode.
The assignee of the present application has previously developed an electrode with significantly improved service life, as described in U.S. Pat. No. 5,023,425, the entire disclosure of which is hereby incorporated by reference, and a method for making such an electrode, as described in U.S. Pat. No. 5,097,111, the entire disclosure of which is hereby incorporated by reference. The ""425 patent discloses an electrode comprising a metallic tubular holder supporting an emissive element at a front end thereof, and having a relatively non-emissive separator or sleeve surrounding the emissive element and interposed between the emissive element and the metallic holder. The sleeve thereby separates the emissive element from the holder. The ""425 patent describes the sleeve as preferably being composed of silver, which has a high resistance to formation of an oxide. The silver and any oxide thereof which does form are poor emitters, and thus the arc will continue to emit from the emissive element rather than from the sleeve or the metallic holder. Service life is thereby significantly extended.
The ""111 patent discloses a method for making an electrode which includes the step of forming a single cavity in the front face of a cylindrical blank of copper or copper alloy, the cavity including an annular outer end portion for receiving a non-emissive member. In particular, a metal blank of relatively non-emissive material, preferably silver, is formed to substantially fit within the cavity. The non-emissive blank is then metallurgically bonded into the cavity by first inserting a disk of silver brazing material into the cavity, then inserting the non-emissive blank. The assembly is then heated to a temperature only sufficient to melt the brazing material, and during the heating process the non-emissive blank is pressed into the cavity, which causes the brazing material to flow upwardly and cover the entirety of the interface between the non-emissive blank and the cavity. The assembly is then cooled, resulting in the brazing material metallurgically bonding the element into the non-emissive blank. Next, the non-emissive blank is axially drilled and a cylindrical emissive element is force-fitted into the resulting opening. To complete fabrication of the electrode, the front face of the assembly is machined to provide a smooth outer surface, which includes a circular outer end face of the emissive element, a surrounding annular ring of the non-emissive blank, and an outer ring of the metal of the holder.
In addition, the torches described by the ""425 and ""111 patents define a rear cavity that extends forwardly towards the front end of the holder such that the emissive element, non-emissive separator, and a portion of the metallic holder form a cylindrical post extending into the rear cavity. A cooling medium, such as water, is circulated in the rear cavity and about the cylindrical post so that heat is transferred from the arc to the cooling water and out of the torch. More specifically, heat is transferred from the arc through the emissive element, non-emissive separator, the copper holder, and any layers of brazing material therebetween to the cooling water. Although this design allows greater heat transfer compared to having no rear cavity, several materials and material interfaces must be crossed, which decreases efficiency.
One particular design defines a rear cavity wherein the cylindrical post includes no portion of the copper holder so that the silver separator is exposed directly to the rear cavity and cooling water circulated therein. For example, FIG. 10 shown in both the ""425 and ""111 patents discloses a plasma arc torch wherein the holder 16b has a through bore in the lower wall, and the non-emissive insert 32b extends through the bore and is exposed so as to directly contact the cooling water in the rear cavity of the holder. This design is advantageous for two reasons: first, silver has a greater thermal conductivity than copper, which increases the heat transfer between the arc and the cooling water; second, the interface between the silver separator and the copper holder is eliminated, which further improves heat transfer. However, the torch shown in FIG. 10 of the ""425 and ""111 patents is not easily formed in that, in addition to the rear cavity being formed in the holder, the lower wall of the holder is bored out and the non-emissive separator is press fit therein.
Thus, while both the electrode described in the ""425 patent and the method of making an electrode described by the ""111 patent provide substantial advances in the art, further improvements are desired. In particular, one method described by the ""425 and ""111 patents provides boring or drilling out a portion of the non-emissive blank, which is typically silver, along a central axis so that the emissive element or insert can be press-fitted therein. While providing a close-fitting relationship between the emissive element and the non-emissive separator, this method disadvantageously results in a loss of silver drilled from the separator to accommodate the emissive element.
Another method used in forming conventional torches provides securing the emissive element in the non-emissive blank or separator by way of brazing. According to this method, the temperature of the silver alloy brazing material must be above its melting point, and thus the temperature of the silver or silver alloy separator is raised almost to its melting point, which can soften the separator material. If this approach were tried in connection with the embodiment of FIG. 10 of the ""425 patent or ""111 patent, however, the softened silver separator may be unable to adequately radially restrain the emissive element when inserted into the silver separator, which could result in the emissive element being xe2x80x9coff-centerxe2x80x9d relative to the central longitudinal axis of the electrode.
The present invention was developed to improve upon conventional electrodes and methods of making electrodes, and more particularly electrodes and methods of making electrodes disclosed in the above-referenced ""425 and ""111 patents. It has been discovered that the difficulties of the electrodes described above, namely the loss of silver from the relatively non-emissive separator and the positioning of the emissive element along the central longitudinal axis of the electrode, can be overcome by positioning the emissive element in the metallic holder before the separator is installed. In one advantageous embodiment, the present invention provides an electrode and method of making an electrode having an emissive element and a generally non-emissive separator disposed in a front cavity defined by the metallic holder, whereby a brazing material is disposed therebetween such that the emissive element""s position along the central longitudinal axis is not affected by the brazing process. In another embodiment, the present invention provides an electrode and method for making an electrode wherein the metallic holder also defines a rear cavity that is sized so that a portion of the separator is exposed to the rear cavity, which thereby improves heat transfer between an arc and a cooling fluid circulated in the rear cavity.
More particularly, in accordance with one preferred embodiment of the invention, an electrode for supporting an arc in a plasma arc torch comprises a metallic holder having a front end and rear end, the front end defining a front cavity. A generally non-emissive separator is positioned in the front cavity and includes an inner peripheral wall. An emissive element is also positioned in the front cavity and includes an outer peripheral wall that is only partially surrounded by the inner peripheral wall of the separator. According to one embodiment, part of the brazing material is disposed between the emissive element and the separator, and also between the separator and the metallic holder. The brazing layer has a melting temperature no greater than the melting temperature of the separator. Thus, the separator and emissive element are metallurgically bonded together such that separator totally separates the emissive element from contact with the outer surface of the metallic holder.
The separator which surrounds the emissive element is preferably composed of a metallic material, such as silver, which has a high resistance to the formation of an oxide. This serves to increase the service life of the electrode, since the silver and any oxide which does form are very poor emitters. As a result, the arc will continue to emit from the emissive element, rather than from the metallic holder or the separator, which increases the service life of the electrode.
In one embodiment, the rear end of the metallic holder defines a rear cavity that extends towards the front end of the holder to expose the separator. The rear cavity can be formed by trepanning or other types of machining, and the exposed separator provides an improved medium for heat transfer from the arc to the cavity, particularly if a cooling medium, such as water, is circulated in the cavity while the torch is in operation.
The present invention also includes a method fabricating the above-described electrode which comprises the steps of forming a front cavity in a generally planar front face of a metallic blank and fixedly securing an emissive element in the front cavity. A relatively non-emissive separator is then positioned in the front cavity of the metallic holder such that the separator is interposed between and separates the metallic holder from the emissive element at the front face of the holder. In one embodiment, the separator has a tubular shape and sized such that the separator and the emissive element have a close-fitting relationship. In addition, the emissive element and separator can be brazed together using a brazing material, such as silver.
Preferably, the front face of the metallic holder is then finished to form a substantially planar surface which includes the metallic holder, the emissive element, and the separator. In one embodiment, a rear cavity is formed in the rear face of the metallic holder such that the separator is exposed to the cavity. In this regard, the metallic holder is trepanned or machined to remove a portion of the holder to thereby expose the separator, which improves the heat transfer from the arc to the cavity. Water or other cooling medium can be circulated within the cavity to further conduct and remove heat from the electrode.
Thus, the electrode of the present invention provides an electrode and method of making an electrode having improved heat transfer properties over conventional plasma arc torches. By positioning the emissive element in the metallic holder before the separator is installed, the position of the emissive element is not affected by a subsequent brazing process. In addition, by exposing the silver separator by trepanning or machining the rear cavity, the front end of the holder is not required to be bored out and the silver separator press fitted therein.